The adiabatic electron affinity of adenine/thymine, AT, is > 1.1 eV from the onset in the anion photoelectron spectrum. We demonstrate that the Watson Crick hydrogen bonded base pair is consistent with this onset using the experimental adiabatic electron affinities of adenine, 1.08(5) eV, and thymine, 0.93(5) eV, and the quantum mechanically predicted electron affinity of AT, 1.4 eV. This differs from the literature assignment to a non-biologically significant proton transferred form. The anion spectra of thymine and hydrated thymine are compared with that of AT. Herschbach Ionic Morse Person Empirical Curves are calculated for thymine anions. (c) 2006 Elsevier B.V. All rights reserved.